Polyesters, in particular, polyethylene terephthalate, having excellent mechanical and chemical properties have widely been used not only in clothes and industrial fibers, but also in magnetic tapes, photographic films, condenser films and molded products such as bottles.
Recently, flame resistance has come to be required in synthetic fibers and in various plastic products from the viewpoint of fire prevention.
Various processes have been proposed for imparting flame resistance to polyesters. Among these processes, the addition of a phosphorus compound to a polyester is considered to be quite effective.
The addition of a phosphorus compound to a polyester, however, generally involves problems such as: (1) the polyester tends to gel; (2) the addition of the phosphorus compound lowers the melting point and the glass transition temperature of the polyester; (3) the ultimate retention ratio of the phosphorus compound is low; (4) the color tone of the polyester is deteriorated; (5) a toxic gas is liable to be evolved in the processes of spinning, stretching, and molding the polyester; etc.
U.S. Pat. No. 4,157,436 discloses a process for producing a fire resistant polyester by copolymerization with a specific phosphinic acid compound. Although this process solves the above described problems to a certain extent, the phosphinic acid compound is expensive, resulting in high polyester production cost due to the addition of such compound in an amount imparting sufficient flame resistance.
JP-A-62-172017 (corresponding to U.S. patent application Ser. No. 438,129 filed on Nov. 20, 1989) discloses a process for producing a flame resistant polyester in which an unsaturated compound is incorporated in a polyester by copolymerization and this product subsequently reacted with a specific phosphinic acid compound. This process allows the production of a flame resistant polyester at a relatively low cost. However, since a phosphorus compound having an active PH bond is reacted with the polyester at high temperature, the process involves the disadvantages that the resulting polyester exhibits a three-dimensional structure to some extent depending on reaction conditions and this lowers the workability thereof upon molding or spinning into fibers or films, or deteriorates the properties of the resulting fibers and films.